Selective synthesis of epichlorohydrin: Via liquid-phase allyl chloride epoxidation over a modified Ti-MWW zeolite in a continuous slurry bed reactor

Article abstract of DOI:10.1039/d0nj04491a

The epoxidation of allyl chloride (ALC) to epichlorohydrin (ECH) with H2O2 using a piperidine (PI)-modified Ti-MWW catalyst (Ti-MWW-PI) in a continuous slurry reactor was investigated to develop an efficient reaction system for the corresponding industrial process. The reaction parameters, including solvent, reaction temperature, t-butanol/ALC mass ratio, ALC/H2O2 molar ratio, weight hourly space velocity of H2O2, and the addition amount of ammonia, were studied in detail to pursue high H2O2 conversion and ECH selectivity. A long catalytic lifetime of 244 h was achieved at high H2O2 conversion (>97.0%) and ECH selectivity (>99.8%) under optimized reaction conditions. The crystallinity was well maintained for the deactivated Ti-MWW-PI catalyst, which was regenerated by a combination of calcination and piperidine treatment. This journal is

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Selective synthesis of epichlorohydrin via
liquid-phase allyl chloride epoxidation over
a modified Ti-MWW zeolite in a continuous
slurry bed reactor†
Cite this:DOI: 10.1039/d0nj04491a
Luoyi Ding, Jinpeng Yin, Wen Tong, Rusi Peng,
Peng Wu
Jingang Jiang, Hao Xu* and
*
The epoxidation of allyl chloride (ALC) to epichlorohydrin (ECH) with H2O2 using a piperidine (PI)-modified
Ti-MWW catalyst (Ti-MWW-PI) in a continuous slurry reactor was investigated to develop an efficient
reaction system for the corresponding industrial process. The reaction parameters, including solvent,
reaction temperature, t-butanol/ALC mass ratio, ALC/H2O2 molar ratio, weight hourly space velocity of
H2O2, and the addition amount of ammonia, were studied in detail to pursue high H2O2 conversion and
ECH selectivity. A long catalytic lifetime of 244 h was achieved at high H2O2 conversion (497.0%)
and ECH selectivity (499.8%) under optimized reaction conditions. The crystallinity was well maintained
for the deactivated Ti-MWW-PI catalyst, which was regenerated by a combination of calcination and
piperidine treatment.
Received 8th September 2020,
Accepted 26th November 2020
DOI: 10.1039/d0nj04491a
rsc.li/njc
later developed transition metal atoms introduced into the
zeolite framework, such as Ti,9 Sn,10 and Fe.11 Titanium-
1 Introduction
Epichlorohydrin (ECH) is an important organic raw material, substituted silicalite-1 (TS-1) with MFI topology was first
which is mainly used in the production of epoxy resins, synthesized by Taramasso and co-workers in 1983.12 It shows
glycerol, and chlorohydrin rubbers.1–3 Annually, the production unique catalytic properties by using hydrogen peroxide as the
of epichlorohydrin increases at a constant rate. Industrially, the oxidant in liquid-phase selective oxidation reactions, including
main methods to produce epichlorohydrin are the chlorination phenol hydroxylation, cyclohexanone ammoximation, olefin
of propylene at high-temperature, the allyl alcohol method, and epoxidation, etc., which aroused research interests due to the
the glycerol method.4,5 However, they all have certain dis- advantages of mild reaction conditions and yielding only water
advantages, such as the equipment corrosion caused by chlorine as the main by-product.13–16 As a second-generation Ti-zeolite,
gas, high cost, complicated processes, and the low atomic economy Ti-MWW possesses two independent sinusoidal 10-membered
due to technical shortcomings. In addition, the above methods ring (MR) channels, as well as 12-MR supercages and side
would produce by-product calcium chloride or chlorine-containing pockets.17–19 With a unique pore structure, Ti-MWW is superior
wastewater, which caused serious environmental pollution. There- to TS-1 in a series of catalytic oxidation reactions, such as the
fore, the development of an environmentally friendly industrial ammoxidation of aldehydes and ketones,20,21 the epoxidation
process for producing ECH has a great practical significance.
Zeolites, having uniform pore structures, good thermal
of olefins22,23 and the oxidative desulfurization of thioethers.24
In recent decades, numerous post-treatments on Ti-MWW
and hydrothermal stability, and unique selectivity, are widely have been carried out to further improve the catalytic perfor-
used as catalysts and adsorbents in petrochemical industries, mance. The strategies of adjusting the pore structure were
environmental protection, bioengineering, food industries, and proposed to enhance the diffusion of substrates and the
pharmaceutical fields.6–8 As catalysts, the catalytic active sites accessibility of Ti-active sites, such as the interlayer-expanded
originate from both the traditional framework Al atoms and the IEZ-Ti-MWW25 and the delaminated Del-Ti-MWW.26 Modifying
the micro-environment of the Ti-active sites could also achieve
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
the purpose of improving the catalytic activity and product
selectivity. For instance, the epoxidation performance of olefins
could be improved by embedding fluorine into the Ti-MWW
Chemistry and Molecular Engineering, East China Normal University, North
Zhongshan Road 3663, Shanghai 200062, China. E-mail: hxu@chem.ecnu.edu.cn,
pwu@chem.ecnu.edu.cn
framework due to the electron-pulling effect of fluorine atoms
on adjacent Ti ions.27 In addition, it is widely recognized that
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04491a
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the framework Ti species with higher coordination number have at 373 K for 12 h after a thorough washing with distilled water.
better catalytic performance in olefin epoxidation reactions.28,29 Finally, the sample was calcinated at 823 K for 6 h to obtain the
Numerous studies have been done on the construction of Ti-MWW catalyst.
the hexa-coordinated Ti species for the purpose of high
2.2.2 Synthesis of Ti-MWW-PI catalyst. According to the
activity. Xu et al. synthesized TS-1 with highly catalytically previous literature,31 starting with Ti-MWW as the parent, the
active hexa-coordinated Ti species, which exhibited a turnover inorganic–organic hybrid Ti-MWW-PI zeolite was hydrother-
number nearly 70% higher than that of the conventional TS-1 mally synthetized with a molar ratio of 1.0SiO2:0.4PI:15H2O
in the epoxidation of 1-hexene.30 The structural rearrangement in a Teflon lined stainless steel autoclave, and this process was
of Ti-MWW in aqueous piperidine (PI) solution, producing carried out in a convection oven with a rotatory speed of 10 rpm
a two-dimensional organic–inorganic miscellaneous MWW-type at 443 K for 24 h. The solid product was dried at 373 K for 12 h
titanosilicate, fabricated hexa-coordinated Ti species with after washing.
PI molecules as the ligand, exhibiting high activity in the
2.2.3 Synthesis of other titanosilicates. TS-1 was synthe-
cyclopentene epoxidation reaction.31 Considering the appealing sized according to previous reports.12 Before calcination, the
catalytic performance of Ti-MWW in the alkene epoxidation synthesized TS-1 needed to be treated in 1.0 M HCl aqueous
reactions and its modifiable structure and micro-environment solution in order to remove the extra-framework Ti species.
of Ti-active sites, it has high potential to serve as an active Using an atom-planting method, Ti-MOR was prepared with the
catalyst in producing epichlorohydrin via the epoxidation of gas-solid reaction of the deeply dealuminated mordenite and
allyl chloride.
TiCl4 vapor at elevated temperatures.34
In our previous work, Ti-MWW was proved to be an effective
catalyst in the batchwise ALC epoxidation reaction.32 In the
2.2 Characterization methods
present study, we tried to fabricate an effective reaction system The structure and crystallinity of the samples were analyzed
based on the modified Ti-MWW catalyst and the slurry bed using a Rigaku ultima IV X-ray powder diffractometer with a
reactor, which simulates the industrial process. Three different Cu-Ka ray source (l = 0.1541 nm) and a nickel filter. The
titanosilicates were first compared in the ALC epoxidation working voltage and current were 35 kV and 25 mA, respec-
reaction, and the Ti-MWW/H2O2/t-butanol catalytic system tively. Using BaSO4 as the reference, the UV-Vis spectra were
demonstrated a high catalytic activity. After the structural obtained on a Shimadzu UV-2700 spectrophotometer. SEM
rearrangement assisted by PI, the obtained Ti-MWW-PI exhib- images were taken using a Hitachi S-4800 scanning electron
ited higher catalytic activity and ECH selectivity than those of microscope with an acceleration voltage of 3 kV and an
Ti-MWW. The reaction parameters including solvent, reaction acceleration current of 10 mA. The XPS spectra were collected
temperature, mass ratio of t-butanol/ALC, molar ratio of using an Axis Ultra Imaging Photoelectron spectrometer
ALC/H2O2, and weight hourly space velocity (WHSV) of H2O2 (Kratos Analytical Ltd). An inductively coupled plasma-
were studied in detail in a slurry bed reactor, and the stability of atomic emission spectrometer (ICP-AES) was used to deter-
the catalyst was also evaluated under the optimized conditions. mine the amounts of Ti and Si in the zeolites on an IRIS
The reasons for the catalyst deactivation were explored, and Intrepid II XSP after the samples were dissolved in HF
accordingly a certain amount of ammonia was added into solution. A BELSORPs instrument was used to determine
the reaction system to inhibit the epichlorohydrin hydrolysis the surface and pore volume of the samples by N2 physical
reaction and prolong the lifetime of the catalyst. Meanwhile, sorption at 77 K after being dehydrated in a vacuum at
the deactivated catalyst could be regenerated by a combination 573 K for 6 h. The specific surface area was calculated by
of calcination and piperidine treatment.
the Brunauer–Emmett–Teller (BET) method, and the pore
volume and external specific surface area were calculated
by the t-plot method. Netzsch Sta 4049 F3 apparatus
was used to determine the content of organic species in
the zeolites in the air with a heating rate of 10 K minꢀ1 in
the temperature range of 298–1073 K. A Varian VNMRS-400
2 Experimental
2.1 Preparation of catalysts
2.2.1 Synthesis of Ti-MWW catalyst. Ti-MWW was synthe- MB NMR spectrometer was used to record the 29Si MAS NMR
sized according to previous literature.33 The silica sol (30 wt%) spectra at a frequency and spinning rate of 100.54 MHz and 3 KHz,
and the tetrabutyl titanate (TOBT) were used as Si source and respectively. The chemical shift was referred to the standard of
Ti source, respectively. Boric acid and PI were used as the [(CH3)3SiO]8SiO12.
crystallization promoter and organic structure-directing agent,
respectively. The gel mixture with a molar ratio of 1.0SiO2:0.05-
2.3 Catalytic reaction
TiO2:0.67B2O3:1.4PI:19H2O was crystallized in a convection
2.3.1 Batchwise allyl chloride epoxidation. The batchwise
oven (10 rpm) at 443 K for 7 days. Then, the product was reaction was carried out in an autoclave with a Teflon-inner.
collected by filtration, washed thoroughly with deionized In a typical run, the desired amount of catalyst (0.05–0.1 g),
water, and dried at 373 K overnight. After that, the product 10 mmol of H2O2 (30 wt%), 10 mmol of ALC and 5 mL
was refluxed in 2.0 M HNO3 aqueous solution at 413 K for 4 h, of solvent were successively added into the reactor. After
where the mass ratio of solid/liquid was 1 : 30, and it was dried stirring vigorously in a water bath at 333 K for 1–2 h, the
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reaction was stopped by cooling in cold water. A certain the reaction in the slurry bed reactor were calculated using the
amount of internal standard of toluene was added before following formulas:
the catalyst was separated from the liquid mixture. Then, the
0
H2O2
n
ꢀ nH O
H2O2
2
2
supernatant was analyzed by gas chromatography, mean-
while, the amount of remaining H2O2 was titrated with
Ce(SO4)2 solution.
H2O2 conv: ¼
H2O2 eff: ¼
ꢁ 100%
0
n
nECH þ nCPDL
n0H O ꢀ nH O
2.3.2 Continuous allyl chloride epoxidation in the slurry
bed reactor. As shown in Scheme 1, the continuous ALC
epoxidation to ECH was carried out in a special three-necked
glass bottle equipped with magnetic agitators. One end of the
bottle was equipped with a condensation device, the middle
one was used for feeding and the third one was connected with
a glass sand filter to facilitate the separation of catalyst and
liquid-phase reaction mixture. The typical operation process
was as follows: 7 g of the catalyst and 120 mL of t-butanol (TBA)
were added into the glass reactor with a maximum volume of
200 mL, and vigorously stirred at a certain temperature. Then,
the mixed solution of TBA, H2O2 (30 wt%) and ALC was
continuously pumped into the reactor at a constant rate by a
dual-plunger pump. The TBA/ALC mass ratio and ALC/H2O2
molar ratio varied in the range from 2 to 4. As the reaction
continued, the liquid reaction mixture discharged from
the outlet filter was collected regularly and analyzed by gas
chromatography.
ꢁ 100%
2
2
2
2
nECH
0
H2O2
ECH yield ¼
ꢁ 100%
n
nECH
nECH þ nCPDL
ECH sel: ¼
The n0H O and n
ꢁ 100%
refer to the initial and final molar
H2O2
2
2
amounts, respectively. nECH and nCPDL stand for the molar
amounts of ECH and 3-chloro-1,2-propanediol (CPDL), respec-
tively. The H2O2 conv., H2O2 eff., ECH yield, and ECH sel.,
represent the conversion of H2O2, the utilization efficiency of
H2O2, the yield of ECH, and the selectivity of ECH, respectively.
3 Results and discussions
3.1 Screening of the titanosilicate catalysts and solvents
2.3.3 Evaluation of catalytic performance. An Agilent 6890
series GC system equipped with a 5937 network mass selective Since the crystal morphology, pore structure and solvent have
detector was used to identify the specific organic species in the crucial influence on the reaction performance, we first screened
liquid-phase reaction mixture. Using toluene as the internal the types of titanosilicates and solvents for the ALC epoxidation
standard substance, the category and content of organic com- reaction. The XRD patterns of TS-1, Ti-MOR, Ti-MWW and
pounds in the liquid phase were determined using a Shimadzu- Ti-MWW-PI are displayed in Fig. S1 (ESI†). All these Ti-zeolites
2014 gas chromatograph with an FID detector, which was showed well-resolved characteristic diffraction peaks indicating
equipped with a Rtx-Wax gas chromatographic column (30 m). high crystallinity. In addition, the XRD pattern of Ti-MWW-PI
The residual H2O2 after the reaction was determined by the showed the layer-related 001 and 002 diffraction peaks at
titration method with 0.05 M Ce(SO4)2 solution. The results of 2y = 3–71, indicating that piperidine molecules were inserted
Scheme 1 Flow diagram of ALC epoxidation in a continuous slurry bed reactor.
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Table 1 Results of the catalytic ALC epoxidation over titanosilicates in
with the size of 0.2–0.4 mm (Fig. S3c, ESI†), and Ti-MOR showed
irregular crystals with the size of about 0.2–0.5 mm (Fig. S3d,
ESI†). The N2 adsorption–desorption isotherms showed that
these four titanosilicates have the characteristics of micro-
porous structures (Fig. S4, ESI†). The specific surface areas
(SSAs) of TS-1, Ti-MOR and Ti-MWW were similar (440–461 m2 gꢀ1),
and they also exhibited very close micropore volumes of
various solventsa
H2O2 (%)
ALC
ECH
Catalyst
TS-1
Solvent
conv. (%)
sel. (%)
Conv.
Eff.
MeCN
MeOH
TBA
23.4
53.9
36.6
86.2
97.8
94.2
93.5
97.9
49.5
95.3
58.9
94.3
47.3
56.5
62.0
91.3
0.146–0.159 cm3 ꢀ1. The SSA of Ti-MWW-PI decreased from
g
Acetone
461 m2 gꢀ1 to 79 m2 gꢀ1 after piperidine treatment, due to the
pore-filling effect of piperidine in the Ti-MWW framework. In
addition, the Si/Ti ratio of Ti-MWW-PI was maintained after
piperidine treatment (Table S1, ESI†).
Ti-MOR
MeCN
MeOH
TBA
0.2
1.3
0.1
2.9
41.5
93.2
89.2
95.0
13.2
4.8
9.3
1.6
27.8
0.5
Acetone
38.8
7.3
For the titanosilicate/H2O2 catalytic systems, the participa-
tion of solvents is indispensable, because it not only homo-
genized the reaction system, but also interacted with the
reactants in various ways. The proper solvent can significantly
accelerate the main reaction and simultaneously effectively
suppress the occurrence of side reactions. In addition, the
optimal solvents for titanosilicates with distinct topological
structures are very different.36 Therefore, the effects of the
zeolite structure and solvents on the epoxidation of ALC to
ECH were investigated in a batchwise reactor. Table 1 sum-
marizes the reaction results of the epoxidation of ALC
over different titanosilicates in various solvents. In this
reaction system (Scheme 2), the main product was ECH, and
the 3-chloro-1,2-propanediol (CPDL) was the main by-product,
Ti-MWW
MeCN
MeOH
TBA
59.2
14.9
90.3
71.8
82.2
99.7
98.6
93.3
99.0
96.5
85.4
43.1
92.1
79.6
94.0
69.3
34.5
98.1
90.2
87.4
TBAb
Acetone
Ti-MWW-PI
MeCN
MeOH
TBA
66.1
84.5
91.4
90.0
90.5
99.7
98.9
99.5
99.8
99.9
94.6
89.6
92.0
92.0
93.0
69.9
94.3
99.4
97.8
97.3
TBAb
Acetone
a
Reaction conditions: catalyst, 0.1 g; allyl chloride, 10 mmol; H2O2
b
(30 wt%), 10 mmol; solvent, 5 mL; temp., 333 K; time, 2 h. The
amount of catalyst and reaction time were halved and other conditions
remained unchanged.
into MWW zeolite and the 3-dimensional (3D) MWW structure which was produced by hydrolyzing ECH at the weak acid sites
was transformed to a 2D one.31 The coordination states of (Ti–OH and Si–OH) of Ti-zeolite. ECH may undergo further
Ti species are shown in Fig. S2 (ESI†). The four titanosilicates solvolysis to produce high-boiling by-product hydroxyethers
showed a dominant band at about 210 nm, which illustrated if methanol was used as a solvent.32 Ti-MOR with a one-
that Ti species mainly tetrahedrally coordinated in the dimensional straight channel structure exhibited the lowest
framework.35 Generally, the bands in the range of 260–290 nm ALC conversion and ECH selectivity for the epoxidation of ALC,
are ascribed to hexa-coordinated Ti species.28 Ti-MWW-PI no matter in which solvent. Furthermre, previous reports also
showed a shoulder peak at 280 nm in addition to the main proved that it was not a suitable candidate for catalyzing the
absorption peak at 210 nm, indicating the presence of hexa- alkene epoxidation.37 On the contrary, it shows extremely high
coordinated Ti species in Ti-MWW-PI. The crystal morpholo- catalytic performance in the ammoximation reactions.38,39
gies of these titanosilicates were revealed by SEM images, as Generally, TS-1 could achieve high alkene conversion in metha-
shown in Fig. S3 (ESI†). Ti-MWW was composed of nano-sheets nol for the alkene epoxidation reaction. However, the TS-1/
with a thickness of B100 nm (Fig. S3a, ESI†). After piperidine MeOH catalytic system showed a moderate catalytic activity for
treatment, the crystal morphology was well maintained the epoxidation of ALC with a conversion of 53.9%. Moreover,
(Fig. S3b, ESI†). TS-1 showed uniform nanoparticle crystals the solvent reaction between methanol and ECH resulted in a
Scheme 2 Reaction routes of Ti-MWW-PI/H2O2-catalyzed ALC epoxidation.
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low ECH selectivity of 94.2%, consistent with previous studies.40
In general, for the alkene epoxidation, the suitable solvent for
Ti-MWW was acetonitrile.32 With t-butanol (TBA) as the solvent,
Ti-MWW showed the highest ALC conversion of 90.3%, but a
relatively low ECH selectivity of 93.3%. Acetone was also a good
solvent for Ti-MWW in the epoxidation of ALC to ECH, but a
previous study proposed that acetone was toxic.41 After the
modification of PI molecules, the ALC conversion was increased
to 91.4%, and the ECH selectivity was up to 99.5% with TBA as the
solvent. Ti-MWW-PI could still maintain the high ALC conversion
when the amount of catalyst and reaction time were halved.
Moreover, no matter which solvent was used, the selectivity of
ECH with Ti-MWW-PI as a catalyst was more than 99.5%, and it
was much higher than that of Ti-MWW. However, when we directly
added piperidine with the same amount as that occluded in
Ti-MWW-PI to the reaction system, the ALC and H2O2 conversion
were decreased from 71.8% and 79.6% to 11.7% and 30.2%,
respectively (Table S3, ESI†). It indicated that the introduction
of PI directly in the reaction system suppressed the reaction
activity, and the PI molecules might need to be embedded in the
Ti-MWW framework to improve the catalytic performance. As
has been proved in our previous study,31 the alkaline PI molecules
not only coordinated with a Ti atom to enhance the catalytic
activity, but also neutralized the weak acidity and prohibited the
occurrence of ring-opening side reactions. In summary, the system
of Ti-MWW-PI/H2O2/TBA was the most suitable for the epoxidation
of ALC to ECH.
Fig. 1 Effect of the reaction temperature on the epoxidation of ALC over
Ti-MWW-PI. Reaction conditions: catalyst, 7 g; temp., 303–333 K; ALC/
H2O2 molar ratio, 3; TBA/ALC mass ratio, 2; WHSV(H2O2) = 0.26 hꢀ1
,
WHSV(ALC) = 1.75 hꢀ1, and WHSV(TBA) = 3.5 hꢀ1
.
the side reactions. Considering the above results, the optimal
reaction temperature of ALC epoxidation was 323 K.
3.2.2 Effect of TBA/ALC mass ratio. The influence of the
solvent amount on the epoxidation of ALC was investigated
with the TBA/ALC mass ratio varied in the range of 2–4.
As shown in Fig. 2, the conversion of H2O2 could reach
99.5%, together with an ECH selectivity of 99.5% when the
TBA/ALC mass ratio was 2. And the conversion of H2O2 and
3.2 Effects of the reaction conditions on continuous
epoxidation of ALC over Ti-MWW-PI
3.2.1 Effect of reaction temperature. As mentioned above,
the Ti-MWW-PI/H2O2 system showed high efficiency in the
batchwise ALC epoxidation with TBA as the solvent; so the
influence of reaction conditions on continuous ALC epoxida-
tion was discussed in detail in the slurry bed to establish
a potential industrial reaction system. Considering that the
boiling point of ALC was low (B318 K), it would accelerate the
volatilization of ALC under normal pressure when the reaction
temperature was much higher than the boiling point. Thus, the
study of ALC epoxidation was carried out in the temperature
range of 303–333 K. As shown in Fig. 1, the conversion of H2O2
did not reach a high level when the temperature was 303 K.
Nevertheless, the ECH selectivity was ultra-high due to the low
ALC conversion. In addition, the catalytic system showed an
induction period with a low catalytic activity at the beginning of
the reaction. The induction period of the reaction was shor-
tened when the temperature was increased from 303 K to 313 K,
meanwhile, the initial H2O2 conversion increased from 96.0%
to 97.6%. The induction period was further shortened when the
temperature was raised to 323 K, and the conversion of H2O2
could reach 499.1%, together with the ECH selectivity of
99.7%. At a higher reaction temperature of 333 K, the utiliza-
tion efficiency of H2O2 decreased to o80% and the selectivity of
ECH was reduced to 99.1%, although the conversion of H2O2
could be further slightly improved, because raising the tem-
perature could accelerate not only the main reaction but also
Fig. 2 Effect of the TBA/ALC mass ratio on the epoxidation of ALC over
Ti-MWW-PI. Reaction conditions: catalyst, 7 g; temp., 323 K; ALC/H2O2
molar ratio, 3; WHSV(H2O2) = 0.26 hꢀ1, and WHSV(ALC) = 1.75 hꢀ1
.
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ECH selectivity remained unchanged when the ratio increased reaction of H2O2 are a pair of competitive reactions, and the
to 3. However, the utilization efficiency of H2O2 suffered a appropriate WHSV of H2O2 was conducive to inhibit the invalid
significant drop. Continuing to increase the TBA/ALC mass decomposition of H2O2 and improve the utilization efficiency of
ratio, the conversion and utilization efficiency of H2O2 tended H2O2.42,43 The proposed reaction pathways of H2O2 in the
to decrease correspondingly, probably due to the dilution effect epoxidation of ALC are summarized in Scheme S1 (ESI†). As
of excess solvent. Therefore, the TBA/ALC mass ratio of 2 was shown in Fig. 4, when the WHSV of H2O2 was 0.2 hꢀ1, the H2O2
the appropriate feed ratio for the epoxidation of ALC.
conversion could reach 99.4%, but the utilization efficiency of
3.2.3 Effect of the ALC/H2O2 molar ratio. As the reactant, H2O2 was only 86.2%, indicating that H2O2 underwent serious
the ALC amount would affect the performance of epoxidation. invalid decomposition. When the WHSV of H2O2 was increased
In general, the epoxidation of ALC was carried out with excess from 0.2 to 0.26 hꢀ1, the H2O2 conversion was nearly
ALC to fully consume H2O2. In the industrial process, the unchanged, while the H2O2 utilization efficiency increased to
unreacted H2O2 and ALC need to be collected for recycling, 93.4%. Further increasing the WHSV of H2O2 to 0.4 hꢀ1, the
and the excessive H2O2 was likely to cause explosion in the utilization efficiency of H2O2 significantly decreased to 78.6%
separation process. To reduce energy consumption in the and the conversion of H2O2 slightly decreased. A higher WHSV
recycling process, it is preferable to use a stoichiometric of H2O2 also enhanced the amount of feeding mixture, which
amount of ALC as much as possible, while ensuring that the exceeded the treating capacity of the catalyst and resulted in
H2O2 is consumed fully. Therefore, it is of great importance to excess H2O2, further aggravating the invalid decomposition of
explore the appropriate ALC/H2O2 molar ratio in the epoxidation of H2O2. Thus, the appropriate WHSV of H2O2 was set as 0.26 hꢀ1
ALC. Keeping the weight hourly space velocity of H2O2 constant, the to ensure the utilization efficiency of H2O2.
ALC/H2O2 molar ratio was varied in the range of 2–4 by changing
3.2.5 Comparison of ALC epoxidation of between Ti-MWW
the weight hourly space velocity of ALC (Fig. 3). Increasing the ALC/ and Ti-MWW-PI. In order to further clarify the effect on
H2O2 molar ratio from 2 to 3 improved the ECH selectivity from catalytic performance after structural rearrangement in the
97.1% to 99.5% and the H2O2 utilization efficiency from 81.6% to continuous ALC epoxidation reaction, Ti-MWW and Ti-MWW-
93.8%, respectively. Further increasing the ALC/H2O2 molar ratio to PI were used to catalyze the epoxidation of ALC to ECH in the
4, both the ECH selectivity and the utilization efficiency of H2O2 slurry bed under the same experimental condition. As shown in
remained unchanged. Considering the residual hydrogen peroxide Fig. 5A, Ti-MWW had a high activity at the initial stage with the
may cause explosion during the separation process, as well as the H2O2 conversion of 99.3%, but the ECH selectivity was only
energy consumption in the process of recycling, the ALC/H2O2 90.0%. Unfortunately, the catalyst was deactivated after 8 h.
molar ratio of 3 was chosen as the feeding ratio for the epoxidation Ti-MWW-PI showed an ultra-high catalytic activity in the
of ALC in the slurry bed reactor.
ALC epoxidation reaction with H2O2 conversion of B99.6%
3.2.4 Effect of H2O2 WHSV. Previous reports showed that and ECH selectivity of B99.6%. In addition, the lifetime of the
the epoxidation reaction of ALC and the invalid decomposition catalyst was significantly increased by at least 5 times. (Fig. 5B).
Fig. 3 Effect of the ALC/H2O2 molar ratio on the epoxidation of ALC over
Fig. 4 Effect of WHSV of H2O2 on the epoxidation of ALC over Ti-MWW-
PI. Reaction conditions: catalyst, 7 g; temp., 323 K; ALC/H2O2 molar ratio,
3; TBA/ALC mass ratio, 2.
Ti-MWW-PI. Reaction conditions: catalyst, 7 g; temp., 323 K; TBA/ALC
mass ratio, 2; WHSV(H2O2) = 0.26 hꢀ1
.
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with a high H2O2 conversion of B99.0% for 48 h at 313 K, and
afterwards the H2O2 conversion began to decrease. Then, the
reaction temperature was increased step by step to maintain a
high H2O2 conversion (497.7%) until the temperature was
raised to 323 K. As has been revealed above, further increasing
the reaction temperature would decrease the utilization effi-
ciency of H2O2. In the temperature range of 313–323 K, the
lifetime of the Ti-MWW-PI catalyst achieved 196 h and a high
ECH selectivity of 499.0% was maintained within 5–204 h. The
average conversion and utilization efficiency of H2O2 reached
97.7% and 91.6% within 5–196 h, respectively, and then
the conversion and utilization efficiency of H2O2 decreased
obviously to 96.5% and 83.0% at 204 h, respectively.
3.3 Catalyst regeneration
Generally, the main reasons for the deactivation of zeolite
catalysts are structural collapse, loss or poisoning of active
components, which is the irreversible deactivation, while
the coke deposition belongs to reversible deactivation. The
physicochemical properties of the used Ti-MWW-PI in the
continuous reaction as shown in Fig. 6 were characterized
to explore the deactivation reason and then perform corres-
ponding regeneration. As shown in Fig. 7c, the intensity of the
layer-related 001 and 002 diffraction peaks decreased signifi-
cantly for the deactivated Ti-MWW-PI, probably due to the coke
deposition and the loss of some piperidine molecules in the
epoxidation reaction of ALC to ECH. The peak intensity for the
regenerated Ti-MWW-PI could be recovered obviously by a
combination of calcination and treatment with piperidine
(Fig. 7d). Additionally, the SEM images showed that the mor-
phology and thickness of all the samples were not changed
obviously, before and after the deactivation (Fig. S5, ESI†).
Thus, the loss of piperidine molecules might be one of the
reasons for catalyst deactivation.
Fig. 5 Comparison of the epoxidation of ALC between Ti-MWW (A) and
Ti-MWW-PI (B). Reaction conditions: Ti-MWW or Ti-MWW-PI, catalyst, 7 g;
temp., 323 K; ALC/H2O2 molar ratio, 3; TBA/ALC mass ratio, 2;
WHSV(H2O2) = 0.26 hꢀ1, WHSV(ALC) = 1.75 hꢀ1, and WHSV(TBA) = 3.5 hꢀ1
.
As mentioned above, the presence of piperidine ligands in
Ti-MWW-PI could prevent the occurrence of the ring-opening
side reaction of ECH, and suppressed the formation of coke
deposition, which might be the reason for the enhanced life-
time of the catalyst.
3.2.6 Evaluation of the lifetime of the Ti-MWW-PI catalyst
in ALC epoxidation. Based on the reaction conditions discussed
above, we evaluated the stability of Ti-MWW-PI under opti-
mized conditions. As shown in Fig. 6, the reaction proceeded
Fig. 6 The lifetime of the epoxidation of ALC with H2O2 over Ti-MWW-PI.
Reaction conditions: Ti-MWW-PI, catalyst, 7 g; temp., 313–323 K; ALC/
H2O2 molar ratio, 3; TBA/ALC mass ratio, 2; WHSV(H2O2) = 0.26 hꢀ1
,
Fig. 7 XRD patterns of Ti-MWW (a), fresh Ti-MWW-PI (b), deactivated
Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
WHSV(ALC) = 1.75 hꢀ1, and WHSV(TBA) = 3.5 hꢀ1
.
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Table 2 The physicochemical properties of various Ti-MWW samples
SSAb (m2 gꢀ1
Total
)
Pore volume (cm3 gꢀ1
)
Weight lossh (wt%)
Catalyst
Si/Tia
Internal
Totalc
Micro.d
Step 1
Step 2
Total
Ti-MWW
39.1
39.5
49.0
48.2
461
79
69
375
14
5
0.636
0.427
0.512
0.662
0.152
0.010
0.001
0.008
—
9.0
—
—
5.6
—
—
Fresh Ti-MWW-PIe
Deactivated Ti-MWW-PIf
Regenerated Ti-MWW-PIg
14.6
39.3
13.4
103
16
8.3
5.1
a
b
c
Determined by ICP analysis. SSA: specific surface area, determined by N2 adsorption isotherms. Total pore volume, given by N2 adsorption at
d
e
f
p/p0 = 0.99. Micropore volume given by t-plot method. Piperidine treatment at 443 K for 24 h. Fresh Ti-MWW-PI after being used in the
continuous ALC epoxidation for 204 h as shown in Fig. 6. Regenerated by a combination of calcination at 823 K for 6 h and piperidine treatment
at 443 K for 24 h. Given by TG analysis, step 1, 423–573 K, and step 2, 573–1073 K.
g
h
To further explain that the loss of PI during the catalytic X-photoelectron spectroscopy (XPS). As shown in Fig. 10b, after
reaction was one of the reasons of deactivation, the 29Si piperidine treatment, the Ti 2p3/2 peak shifted from 460.3 and
MAS NMR spectra were collected. Ti-MWW showed a resonance 459.3 eV to 459.7 and 458.9 eV, respectively, indicating that the
band at ꢀ98.1 ppm, attributed to the Q3 silanol groups. density of the electron cloud around the Ti species increased
Compared with the Ti-MWW, the resonance bands of fresh and the coordinated state of the Ti species became more
Ti-MWW-PI at ꢀ102.6 ppm (Q3 groups) and ꢀ94.8 ppm negatively charged, which implied the presence of hexa-
(Q2 groups) were very intense, indicating that there were more coordinated Ti species with piperidine (PI) as the ligand,
hydroxyl groups due to the transformation of the 3D MWW consistent with the previous report.31 For the deactivated
structure to a 2D lamellar one as well as the dissolution of Ti-MWW-PI, the binding energy of the Ti 2p peak was increased
MWW layers caused by PI molecules. After deactivation, the (Fig. 10c), mainly due to the loss of PI during the reaction, in
amounts of Q2 and Q3 groups significantly decreased, probably good agreement with the XRD pattern (Fig. 7c) and the 29Si MAS
because the 2D structure was transformed back into the 3D NMR spectra (Fig. 8c). After the calcination and piperidine
structure due to the loss of PI molecules during the reaction treatment, the binding energy of regenerated Ti-MWW-PI was
process, in agreement with the results of the XRD patterns. restored to the state of fresh Ti-MWW-PI (Fig. 10d).
After calcination and piperidine treatment, the amounts of
Q2 and Q3 groups were obviously restored.
The thermogravimetric curves of Ti-MWW, fresh Ti-MWW-PI,
deactivated Ti-MWW-PI, and regenerated Ti-MWW-PI are shown
The variation of the coordination state of Ti-active sites in Fig. 11. Generally, the weight loss below 423 K is attributed to
before and after the reaction was monitored by the UV-Vis
spectra (Fig. 9). The Ti-MWW related samples all showed a
dominant band at B210 nm, indicating that the Ti species
mainly existed in the tetra-coordinated state.35 After rearrange-
ment of Ti-MWW, a shoulder peak appeared at 280 nm of
Ti-MWW-PI displaying the presence of hexa-coordinated Ti
species.28 The band at 260 nm appeared after the catalyst was
deactivated, probably due to the leaching of Ti species from
the framework to form an extra-framework Ti species in the
catalytic reaction.44 And the band at 330 nm in the UV-Vis
spectrum of the deactivated catalyst could be attributed to the
TiO2 species33 and the coke deposition.31 The 330 nm band
almost disappeared after regeneration, indicating the removal
of coke deposition upon calcination. However, the extremely
small amount of TiO2 species, originating from the migration
of framework Ti to the external surface in the reaction, still
existed after regeneration, which contributed to the minor
adsorption band at 330 nm in the regenerated sample. And
the band at 280 nm corresponding to the hexa-coordinated
Ti species was recovered because of the piperidine treatment. The
Si/Ti ratio of the used Ti-MWW-PI (49.0) was increased compared
to the fresh Ti-MWW-PI (39.5) as shown in Table 2, indicative of
the leaching of Ti-active sites in the reaction, which would
accelerate the deactivation of the catalyst. In addition, the Si/Ti
ratio was maintained in the regeneration process.
Fig. 8 29Si MAS NMR spectra of Ti-MWW (a), fresh Ti-MWW-PI (b),
deactivated Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
To further investigate the coordination environment of
titanium species, the state of Ti species were characterized by
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Furthermore, the total organic species accounted for B39.3%,
implying the coke deposition was the main reason of the
deactivation of the catalyst. Thus, reducing the formation of
by-product would be effective to extend the lifetime of
the catalyst by suppressing the coke deposition.32 The TG curve
of the regenerated Ti-MWW-PI was similar to the fresh
Ti-MWW-PI, meaning that it could be restored after a combi-
nation of calcination and piperidine treatment (Fig. 11b and d).
Part of the introduced piperidine molecules coordinated
with the Ti atoms by N atoms, and the remaining piperidine
molecules filled in the pore channels, because the amount
of piperidine molecules was more than that of Ti species
in Ti-MWW-PI.45 Therefore, it could be speculated that the
amount of PI was related to the amount of Ti species. The CHN
elemental analysis showed that the amount of N in the regen-
erated Ti-MWW-PI (1.77%) decreased slightly compared with
the fresh sample (1.95%). According to ICP analysis, it could
be observed that the Ti leaching occurred in the epoxidation
process, and combining the TG analysis, the specific amount of
Ti species could be determined. By calculating the Ti/PI ratio,
it could be determined that the Ti/PI ratio of fresh Ti-MWW-PI
(0.24) and regenerated Ti-MWW-PI (0.23) were almost unchanged
(Table 3). Thus, the decrease of piperidine content after regenera-
tion was highly related to the leaching of Ti species.
Fig. 9 UV-Vis spectra of Ti-MWW (a), fresh Ti-MWW-PI (b), deactivated
Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
After the rearrangement of the structure, the piperidine
molecules entered the intralayer pores and interlayer spaces
of Ti-MWW, decreasing the specific surface area and micropore
volume of the fresh Ti-MWW-PI. The specific surface area and
micropore volume further decreased for deactivated Ti-MWW-
PI, due to the coke deposition during the reaction process
(Table 2). The specific surface area and micropore volume of
the catalyst can be recovered well after regeneration (Fig. 12).
However, the total surface area (103 m2 gꢀ1) of the regenerated
Ti-MWW-PI was larger than that of the fresh Ti-MWW-PI
(79 m2 ꢀ1), because the amount of piperidine entering the
g
MWW framework was reduced due to the leaching of Ti species,
which had been revealed above. The catalytic performance of
the fresh Ti-MWW-PI, deactivated Ti-MWW-PI, and regenerated
Ti-MWW-PI for the epoxidation of ALC was evaluated in
a batchwise reactor (Table 3). After deactivation, the ALC
conversion and ECH selectivity significantly decreased from
90.0% and 99.8% to 11.0% and 40.1%, respectively. And the
ALC conversion and ECH selectivity of regenerated Ti-MWW-PI
recovered to 78.7% and 99.7%, respectively. The catalytic
performance could only be partially restored due to the leaching
of Ti-active sites and gradual variation of Ti coordination sites.
Fig. 10 Ti 2p XPS spectra of Ti-MWW (a), fresh Ti-MWW-PI (b), deacti-
vated Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
physically adsorbed water. After the structural rearrangement
of Ti-MWW assisted by PI molecules, there were two peaks in
the temperature ranges of 423–573 K and 573–1073 K, attrib-
3.4 Extending the lifetime of a catalyst
3.4.1 Effect of ammonia concentration on ALC epoxidation
uted to the thermal decomposition of piperidine between and over Ti-MWW-PI. Based on the above discussion, the reasons
within the MWW layers, respectively. Meanwhile, a small part for Ti-MWW-PI deactivation were divided into three parts,
of weight loss at high temperature could also be contributed by namely coke deposition, the loss of piperidine molecules,
the condensation of Si–OH groups. The deactivated Ti-MWW-PI and the partial leaching of Ti-active sites. Since the leaching
showed three weight loss peaks in the temperature range of of Ti-active sites and the loss of piperidine molecules were
298–1073 K (Fig. 11c), with the two weight losses above 423 K inevitable, we tried to reduce the amount of coke deposition to
attributed to the piperidine molecules and coke deposition. obtain a longer catalytic life. Previous reports indicated that the
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Fig. 11 TG curves of Ti-MWW (a), fresh Ti-MWW-PI (b), deactivated Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
Table 3 A comparison of ALC epoxidation over the fresh Ti-MWW-PI,
deactivated and regenerated Ti-MWW-PI zeolitea
N amountb ALC
ECH
H2O2
H2O2
Catalyst
(wt%)
conv. (%) sel. (%) conv. (%) eff. (%)
Fresh
1.95
90.0
11.0
78.7
99.8
40.1
99.7
92.0
18.7
90.9
97.8
59.2
86.6
Ti-MWW-PI
Deactivated
Ti-MWW-PIc
Regenerated
Ti-MWW-PId
a
—
1.77
Reaction conditions: catalyst, 0.05 g; allyl chloride, 10 mmol; H2O2
(30 wt%), 10 mmol; TBA, 5 mL; temp., 333 K; time, 1 h. Determined
by CHN analysis. Deactivated Ti-MWW-PI after being used in the
continuous ALC epoxidation for 204 h as shown in Fig. 6. Regenerated
b
c
d
by a combination of calcination and piperidine treatment.
addition of an appropriate amount of ammonia could inhibit
the occurrence of ring-opening reaction effectively.46,47 Con-
sidering the results of the above reactions, the ECH selectivity
could reach more than 99.5% under the optimized conditions.
However, the high level of ECH selectivity may not give an
obvious contrast when adding the ammonia. Therefore, the
effect of ammonia was explored with the molar ALC/H2O2 ratio
of 2 under relatively low ECH selectivity level. As shown in
Fig. 12 N2 adsorption–desorption isotherms of Ti-MWW (a), fresh Ti-
MWW-PI (b), deactivated Ti-MWW-PI (c), and regenerated Ti-MWW-PI (d).
Fig. 13, when the concentration of ammonia was increased Meanwhile, the lifetime of the catalyst was improved signifi-
from 0 ppm to 4 ppm, the conversion of H2O2 was unchanged, cantly, because the weak alkalinity of ammonia neutralized the
and the selectivity of ECH increased from 98.3% to 99.0%. weak acidity from the titanosilicate to a certain extent and
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ammonia must be controlled within a certain concentration
range, otherwise the opposite result would occur.
3.4.2 Comparison of the lifetime after adding ammonia.
The reaction of ALC epoxidation was conducted in the presence
of ammonia in the temperature range of 313–323 K, and other
conditions were the same as those in the previous lifetime
evaluation (Fig. 6). The catalytic lifetime was improved from
196 h to 244 h with a high ECH selectivity (499.8%) and a high
H2O2 conversion (497.0%), demonstrating excellent catalytic
stability (Fig. 14).
4 Conclusions
Herein, the slurry bed reaction process of ALC epoxidation to
ECH was systematacially investigated with the Ti-MWW-PI/
H2O2/TBA catalytic system. Under optimized conditions, the
lifetime of the Ti-MWW-PI catalyst achieved 244 h with a high
ECH selectivity (499.8%) and
a high H2O2 conversion
Fig. 13 Effect of ammonia concentration on the epoxidation of ALC over
Ti-MWW-PI. Reaction conditions: Ti-MWW-PI, catalyst, 7 g; temp., 323 K;
ammonia concentration, 0 ppm (A), 4 ppm (B) and 8 ppm (C); ALC/H2O2
molar ratio, 2; TBA/ALC mass ratio, 2; WHSV(H2O2) = 0.26 hꢀ1, WHSV(ALC) =
(497.0%). The Ti-MWW-PI/H2O2/TBA system catalyzed the
epoxidation of ALC displaying that the main reason for the
deactivation of the catalyst was coke deposition. Meanwhile,
the loss of PI molecules and Ti species during the reaction
process would also accelerate the deactivation of the catalyst.
After a combination of calcination and piperidine treatment,
the catalytic performance of the deactivated catalyst could only
be partially restored due to the leaching of part of the Ti species
during the reaction process. In addition, the employment of
a slurry bed continuous reactor increased the contact area
between the catalyst and the liquid-phase reaction mixture to
promote the catalytic activity of reaction, which also facilitated
the separation of the catalyst. This work might provide a
potential way for the green industrialization of the epoxidation
of ALC to ECH.
1.17 hꢀ1, and WHSV(TBA) = 2.34 hꢀ1
.
suppressed the occurrence of ring-opening side reactions.
However, when the concentration of ammonia continued to
increase to 8 ppm, the conversion of H2O2 and the yield of
ECH dropped from 99.2% and 95.0% to 98.1% and 84.6%
respectively, mainly due to the toxic effect of excess alkaline
ammonia on the catalytic active centers of titanosilicate, which
was consistent with previous reports.48,49 Therefore, in order to
increase the ECH yield and lifetime effectively, the amount of
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study is financially supported by the National Natural
Science Foundation of China (Grant No. 21872052, 21533002,
and 21972044), the Ministry of Science and Technology of the
People’s Republic of China (2016YFA0202804) and the Funda-
mental Research Funds for the Central Universities.
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